A Novel Indium Oxide-Based Nanostructured Material Designed for CO₂ Detection ^†

Abstract

Low-cost sensors working at low/room temperature for CO₂ mapping in indoor–outdoor environments are in growing demand. Solid-state gas sensors are a suitable alternative to expensive optical sensors, but to date, materials designed for chemoresistive devices have not proven functional for CO₂ detection. This work addresses this challenge both in terms of sensing materials research, with the innovative use of alkali metals as dopants in semiconductors, and in terms of deeply understanding the sensing mechanism through DRIFT spectroscopy. The result is a sensor operating at 200 °C that detects CO₂ between 250–5000 ppm with a negligible effect of humidity above 17 RH%.

1. Introduction

Carbon capture and storage is critical to climate change policies and strategies aimed at reducing global warming under the Paris Agreement. To address this challenge, the development of miniaturized devices based on the Internet of Things (IoT) to ensure the mapping of CO₂ is pivotal. Nowadays, the most widely used direct monitoring systems for carbon dioxide are based on non-dispersive infrared (NDIR) sensors. However, compared with chemoresistive sensing devices, they suffer from many drawbacks, such as complexity, a short device lifetime, and being susceptible to interference gases. Therefore, chemoresistive devices represent an interesting alternative to the sensors currently used. In this work, an innovative nanostructured semiconducting powder based on indium oxide and doped with sodium is proposed (Na:In₂O₃) as a functional material for screen-printable chemoresistive CO₂ sensors [1].

2. Materials and Methods

The Na:In₂O₃ nanopowder was synthesized by the sol-gel method and used as a functional material for screen-printed films deposited onto alumina substrates. A complete material characterization was performed from a morphological, structural, chemical, and optical point of view. A deeper electrical investigation was carried out to study the sensing performance towards CO₂ (sensitivity, selectivity, stability, and effect of humidity). Furthermore, Fourier transform infrared diffuse reflectance (DRIFT) spectroscopy was combined with electrical characterization using an operando approach to monitor the gas-solid interaction occurring on the solid-state chemical sensor surface under various environmental conditions [1].

3. Discussion

The sensitivity of Na:In₂O₃ to CO₂ was investigated by exposing the film, operating at 200 °C, to different gas concentrations. Figure 1a shows a high and fast response with completely reversible behavior at a working temperature lower than commonly used for semiconductor-based gas sensors. Na:In₂O₃ film was tested with possible interfering gases to cover different relevant chemical species present in real applications, showing high selectivity (Figure 1b). The impact of water vapor on sensing performance is limited, even at low CO₂ concentrations of 500 ppm (insert Figure 1b). In particular, apart from a partial conductance drop up to 17 RH%, for increasing humidity concentrations, the performance of the In₂O₃-based sensor seems to stabilize. Therefore, electrical activity towards different concentrations of different gases in dry and humid air highlighted high sensitivity and a negligible influence from interfering gases. In addition, in this work, in order to obtain additional information on the surface reaction mechanism of Na:In₂O₃ gas sensors to CO₂, we provided operando DRIFT spectroscopy, which proved the adsorption of CO₂ on the surface of this innovative material, leading to the formation of carbonate species [1,2].

4. Patents

Patent Pending, n° IT 102022000022314.